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Silica is a naturally occurring mineral composed principally of silicon dioxide (SiO2) which exists in both crystalline and amorphous forms. Crystalline refers to the orientation of the SiO2 molecules in a fixed pattern as opposed to a non-periodic, random molecular arrangement which is defined as amorphous (without shape). Quartz, cristobalite and tridymite are the three most common crystalline forms. Silica is used in the fabrication of stone and clay products, glass enamels and ceramic products. It is also found in sandstone, granite, flint, slate, abrasive, brick, concrete, cement mortar, sand and asphalt.
Exposure to silica typically occurs through inhalation on construction or demolition projects, or during industrial processes involving silica. Inhalation of crystalline silica can cause silicosis, bronchitis, fibrosis (scar tissue in the lungs which makes breathing difficult), granulomatosis infections (such as tuberculosis), and lung cancer.
         Regulatory Requirements 
Industrial processes involving silica are regulated under the Ontario Occupational Health & Safety Act, R.S.O. 1990, Regulation 845 as amended by O.Reg. 606/05, Designated Substance – Silica. Exposure levels and respiratory protection in this regulation also apply to construction projects at a workplace where workers are likely to be exposed to silica.
A guideline respecting silica on construction projects was published by the Occupational Health and Safety Branch of the Ontario Ministry of Labour in September 2004. The disturbance of silica on construction projects is classified as Minimum (Type 1), Intermediate (Type 2) or Maximum (Type 3) work, each of which will have defined precautionary measures. Respiratory protection required under the guideline is dependent upon the classification of work. There is no specific requirement to monitor exposure levels.